1. Field of the Invention
The present invention relates to a plastic optical fiber having good light transmission characteristics, which can transmit not only visible rays but also rays having a wavelength in the infrared-to-near infrared region.
2. Description of the Related Art
A conventional plastic optical fiber having a core-sheath structure comprising polymethyl methacrylate, polystyrene or polycarbonate as the core component and a perfluoroalkyl methacrylate polymer as the sheath has a larger diameter, a better flexibility, a larger numerical aperture and a better connecting property to a light source than an optical fiber having a core composed of quartz or multi-component glass, and this plastic optical fiber is widely used as the light transmission medium in the fields of short-distance communication, data banks and light sensors.
When visible light of 400 to 600 nm is used, the attenuation of the core-sheath plastic optical fiber comprising polymethyl methacrylate, polystyrene or polycarbonate as the core component is 150 to 400 dB/km and thus is very satisfactory, but for light having a wavelength longer than about 650 nm, the attenuation is very large. The main wavelength of emission elements now manufactured on an industrial scale, that can be used as the light source in this field, is longer than 700 nm and in the infrared-to-near infrared region. The light transmission characteristics of the conventional plastic optical fibers are not satisfactory when these emission elements are used as the light source.
The polymer constituting the core of the plastic optical fiber, such as polymethyl methacrylate, polystyrene or polycarbonate, has many C-H bonds, and these C-H bonds have an absorption based on the stretching vibration or harmonic absorption at 650 to 660 nm, and this absorption is a cause of the extreme increase of the attenuation of radiation having a wavelength in the infrared-to-near infrared region.
Development of plastic optical fibers capable of transmitting radiation having a wavelength in the infrared-to-near infrared region has advanced and for example, Polymer Preprints, Japan, Vol. 31, No. 9,page 2357 discloses a core-sheath type plastic optical fiber comprising deuterated polymethyl methacrylate as the core, Japanese Unexamined Patent Publication No. 61-114210 discloses a core-sheath type plastic optical fiber comprising an a,b,b-trifluoro-pentafluorostyrene polymer as the core, and Japanese Unexamined Patent Publication No. 61-142211 discloses a core-sheath type plastic optical fiber comprising an a,b,b-trideuteropentafluorostyrene polymer as the core.
Since deuterated polymethyl methacrylate has no C-H bond, the initial attenuation of a core-sheath type plastic optical fiber comprising this polymer as the core is 20 dB/km at a wavelength of 650 to 680 nm and is very small, but if the optical fiber is allowed to stand for more than one month from manufacture, the attenuation is drastically increased and becomes 300 dB/km or more. This is because the water-absorbing property of deuterated polymethyl methacrylate is high, and the absorption of water increases in the plastic optical fiber with the lapse of time. An optical fiber showing such a large change with the passage of time is difficult to handle and cannot be used as an optical fiber having high reliability.
The attenuation of a plastic optical fiber comprising perfluoropolystyrene as the core at a wavelength of 660 nm is 115 to 140 dB/km, but it is reported that the attenuation of near infrared light having a wavelength of 850 nm is 400 dB/km, and the transmission characteristics for radiation having a wavelength in the near infrared region are not completely satisfactory. Furthermore, since the toughness of perfluoropolystyrene is lower than that of polymethyl methacrylate or polycarbonate, the plastic optical fiber comprising perfluoropolystyrene as the core has a problem in that the optical fiber is readily broken and the resistance to handling is not satisfactory.
Although development of a plastic optical fiber comprising a perfluoroalkyl methacrylate polymer as the core has advanced, since the glass transition temperature of this polymer is low and the shapeability to an optical fiber is poor, the preparation of an optical fiber having a uniform diameter is difficult and an optical fiber having good transmission characteristics cannot be obtained from this polymer.